Anthropologist Donald Johanson and student Tom Gray were working as part of an expedition to survey the fossil-rich Hadar Formation in the Afar Depression in Ethiopia. Re-checking a gully on a hunch, Gray noticed an arm-bone fragment. A hurried search soon revealed more bone. The whole expedition was diverted to the site, and for the next three weeks the area was combed for fossil bone. On the first night after the initial discovery, the Beatles’ hit Lucy in the Sky with Diamonds was playing loudly on a tape-recorder in the camp, so the fossil hominin was nicknamed Lucy. Her scientific name is AL 288-1.

A total of 47 bone fragments were recovered, and there were no duplicate parts, making Lucy 40% complete – an astonishing discovery for a discipline that commonly deals in fragments of single bones. Tooth-wear implies that she was about 20 years old at death. She stood just 1.1m tall, and weighed about 29kg. The shape of the pelvis suggests Lucy was female, though some think she may have been a he.

Lucy explained

Of modest size and with short legs, long arms, and small brain-case, Lucy would have looked rather like a modern chimpanzee. But there was a crucial difference: she walked upright. Johanson had previously discovered the lower end of a thighbone and the upper end of a shinbone of another specimen nearby. Fitted together, the angle of the knee-joint clearly showed that this hominin was an upright walker. Lucy was found only 2.5km away, was of similar date, and her pelvis and legs looked like those of modern humans, confirming the impression of bipedalism in the Hadar group of hominins. She probably lived as part of a small group moving around the landscape foraging for fruit, nuts, seeds, eggs, and other food-stuffs.

Lucy interpreted

Lucy belonged to the species Australopithecus afarensis (meaning Southern Ape of Afar). She represents the evolutionary line that separated the hominins from the apes several million years ago. Genetically, chimpanzees are the closest of the living apes to humans, but Lucy was already on a separate evolutionary branch-line when she lived 3.2 million years ago.

Her bipedalism was a revolutionary development. As climate change reduced the forests and replaced them with savannah, Lucy and her bipedal brethren would have been at an adaptational advantage – able to walk longer distances than the average ape. More importantly, bipedalism would have freed arms and hands for tool-making and other forms of labour. This, in turn, would have caused selection in favour of higher brain capacity. Hand and brain, labour and intellect, skill and thought would have begun that explosive interaction whose evolutionary culmination is us, modern Homo sapien sapiens. Lucy and her Australopithecine family and friends are the ones who walked before us.